Production of wood pulp



July v6, 1954 F. c. ALDRED ET AL 2,583,090

PRODUCTION oF woon PULP Filed April 30, 1952 JHM/Kif ma v 2 l j :KfW/5P] /m/enfors AH190' Crow/herA/a'red Geofe Burda/#Tamer By file/'r a fomeys Patented .luly 6, 1954 l f y 2,683,090

IRODUCTION OF WOOD PULP Fred C. Aldred, Coventry, and Geoffrey B. Turner, Green Lane, Coventry, England, assignors to Courtaulds Limited, London, England, a British Company Application April 30, 1952, Serial No. 285,244

Claims priority, application Great Britain June 21, 1951 8 Claims. (Cl. 92--11) This invention relates to the production of Wood pulp.

In the production of wood pulp by the so-called sulphite process, wood chips are cooked with a liquor consisting of an aqueous solution of a bisulphite, usually calcium or magnesium bisulphite, containing excess sulphur dioxide. Although the cooking reaction is complex it is generally accepted that the cooking treatment converts the lignin inthe wood into ligno-sulphonic acid which is then hydrolysed into soluble or readily dispersable aggregates. During the cooking the sulphite liquor tends to decompose with the formation of sulphate and it is believed that some of the known undesirable effects of prolonging a sulphite cook, such as excessive degradation of the cellulose in the wood, are due to sulphate or other products of this decomposition.

It is also known to circulate the sulphite liquor continuously through the wood chips by means of a circulatory system including one or more heat exchangers. The presence of calcium sulphate in the liquor causes scale formation in the pipes and the heat exchangers vand this scaling is difficult to lremove.y TheV difficulty due to scaling is particularly serious when attempts are made to concentrate the Waste liquor prior to further treatment.

The object of the present invention is to inhibit or reduce the formation of sulphate during the cooking.

According to the present invention, in a process for the production of wood pulp by cooking wood chips with an aqueous solution of a bisulphite containing excess sulphur dioxide, the cooking liquor is maintained in contact with copper in order to inhibit or reduce the formation of sulphate during the cooking.

The process of this invention is preferably carried out by continuously circulating the sulphite liquor through a closed system comprising a vessel containing the wood, one or more heaters, a second vessel containing the copper, which may conveniently be in the form of gauze or lings that is to say, in a discrete form having large surface area, and a separating device, for example a lter, for removing copper sulphite from the liquor.V The use of such a system prevents the contamination of the pulp by copper sulphide. The process of Ithe inventionmay however be carried out by heating the wood, the sulphite liquor and copper together in a single vessel.`

A circulatory system suitable for use in the present invention is shown in the accompanying drawing. The wood chips are packed in a vessel l, conveniently made of stainless steel and the sulphite liquor is continuously circulated through the vessel I, through an external heater 2, a pump 3, a second heater 4, a container 5 containing copper, a lter 6 to remove copper sulphide and back into the vessel l. y,

In a modification the container for the wood, the container for the copper and the filter may be built up in a single container having three separate but connected compartments.

Experiments which we have carried out have shown that the overall decomposition of the sulphite liquor is probably in accordance with the following reaction:

We have found that this reaction is catalysed by metals such as iron, cobalt, molybdenum, manganese and zinc, many of which are used in construction materials for the cooking vessels, andby many substances either present in the Wood or formed during the cooking. By maintaining the sulphite liquor in contact with a small amount, for example from 0.05 to 1.0 per cent of copper, based on the weight of the wood chips, in accordance with this invention the sulphate formation is effectively reduced. In addition to the sulphate reduction the present invention in general gives increased yields of dry pulp.

The copper is converted during the cooking into copper sulphide which may be recovered if desired, for example from the lter 6, and reconverted into copper for use in a subsequent cook.

The invention is illustrated by the following examples in which parts and percentages are by weight:

Example 1 1 part of spruce Wood chips, 0.005 part of copper filings and 4.5 parts of a sulphite liquor containing 5 per cent total sulphur dioxide and 1.3 per cent combined sulphur dioxide in the form of magnesium sulphite were cooked in a stainless steel vessel, the temperature being rst raised to centigrade and kept at this point for 3 hours whereupon the temperature was raised over 1/2 hour to 150 centigrade and then kept at this point for 2 hours 40 minutes. The waste liquor from this cook was found to contain 0.06 per cent of sulphate and the yield of dry pulp was 4'7 per cent of the weight of the vdry wood whereas in a similar cook carried out without the addition of the copper filings, 0.13 per cent of sulphate was present in the waste liquor, and the yield of dry wood pulp was 45.4 per cent.

Example 2 In this example a circulatory system as described in the drawing was used.

1 part of spruce Wood chips was packed into the container l and the container 4 was packed with copper gauze. A sulphite liquor containing 6 per cent total sulphur dioxide and 1.2 per cent combined sulphur dioxide in the form of sodium sulphite was, continuously circulated through the system, the ratio of liquor to wood being 4.5 1. The heaters were adjusted so that the temperature of the liquor rose to 110 centigrade in 11/2 hours, this temperature being maintained for 21/2 hours; the temperature was then raised over 11/4 hours to 150 centigrade and this temperature was maintained for 1 hour 40 minutes. The maximum pressure throughout was 120 lb./in.2.

The sulphate content of the liquor at the end of the cook was 0.2 per cent Whereas the ligure for a similar run in the yabsence of copper was within the range of 0.5 to l per cent. The yield in Example 2 was 47 per cent kof dry pulp based on the weight of the dry wood, a yield which is generally higher than that obtained in the absence of the copper.

What we claim is:

l. In a process for the production of wood pulp by cooking wood chips with an aqueous solution of a bisulphite'containing excess sulphur dioxide, the step comprising maintaining the cooking liquor in contact with metallic copper in discrete form whereby the formation of sulphate during the cooking is reduced.

2. In a process for the production of wood pulp by cooking Wood chips with an aqueous solution of a bisulphite containing excess sulphur dioxide, the steps comprising continuously circulating the sulphite liquor through a closed system comprising a vessel containing the wood, at least one heater, a second vessel containing metallic copper in discrete form and a separating device to remove copper sulphide from the liquor before the liquor is passed through the wood pulp 4 whereby the formation of sulphate during the cooking is reduced.

3. The process as claimed in claim 1 wherein the copper is in the form of copper filings.

4. The process as claimed in claim 1 wherein the copper is 'in the form of copper gauze.

5. In a process for the production of Wood pulp by cooking Wood chips with an aqueous solution of a bisulphite containing excess sulphur dioxide, the step which comprises maintaining the cooking liquor in contact with from 0.05 to 1.0% of copper 'based on the weight of the wood chips, said copper being in a discrete physical form having a large surface area whereby formation Vof .sulphate during the cooking is reduced.

6. A process as claimed in claim 5 wherein the sulphite liquor is continuously circulated through a closed system comprising a rst stage wherein the Wood chips are digested with the cooking liquor, a second stage wherein the cooking liquor is heated, a third stage wherein the liquor is contacted with the copper, and a fourth stage wherein the liquor having contacted the copper is treated to remove copper sulphide.

'7. In a process for the production of wood pulp by cooking wood chips with an aqueous solution of a bisulphite containing excess sulphur dioxide, the step which comprises maintaining the cooking liquor in contact with from 0.05 to 1.0% of copper filings based -on the weight of the Wood chips, whereby formation of sulphate during the cooking is reduced.

8. In a process for the production of wood pulp by cooking wood chips with an aqueous solution of a bisulphite containing excess sulphur dioxide, the step which comprises maintaining the cooking liquor in contact with from 0.05 to 1.0% of copper gauze based on the weight of the wood chips, whereby formation of sulphate during the cooking is reduced.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 360,484 Schnurmann et al. Apr. 5, 1887 483,942 Radford Oct. 4, 1892 

1. IN A PROCESS FOR THE PRODUCTION OF WOOD PULP BY COOKING WOOD CHIPS WITH AN AQUEOUS SOLUTION OF A BISULPHITE CONTAINING EXCESS SULPHUR DIOXIDE, THE STEP COMPRISING MAINTAINING THE COOKING LIQUOR IN CONTACT WITH METALLIC COP- 